Silica granules are used in conventional manner to build up an optical fiber preform, in particular when it is desired to manufacture a monomode optical fiber. The build-up operation is performed, for example, on a bench that is fitted with a plasma torch which is fed with silica granules which are projected onto the preform until a desired diameter has been built up.
Granules obtained by agglomerating silica soot particles are of relatively low density and can contain impurities, coming from the soot particles themselves or from the reactor in which agglomeration took place.
It is known to purify and densify granules prior to using them in a bench for building up a preform. At present, the granules are placed in a dense crucible of opaque silica of very high purity so as to avoid any risk of the granules being contaminated by the crucible. Thereafter, the crucible is placed in a furnace containing an atmosphere of inert gas mixed with a chlorine-containing compound and having a temperature lying in the range 1000 degrees Celsius (.degree. C.) and 1500.degree. C. The chlorine-containing compound diffuses into the granules where it reacts with alkaline or metallic impurities, e.g. by forming gaseous chlorides which are then evacuated from the furnace. Under the action of temperature, the granules densify individually. The crucible is removed from the furnace after a length of treatment time that corresponds to the grains beginning to densify between one another, which densification should be minimized.
The known method of purifying and densifying silica granules is not without its drawbacks.
Firstly, the chlorine-containing compound comes into contact with the granules only via the open portion of the crucible, since the dense walls thereof are gasproof. As a general rule, the crucible is cylindrical in shape and has an open top end which forms a surface via which the chlorine-containing compound diffuses amongst the granules. At given partial pressure for the chlorine-containing compound in the gaseous atmosphere of the furnace, the time required for the chlorine-containing compound to diffuse amongst the granules over the full height of the crucible turns out to be quite long.
Also, although the pure silica crucible does not present any risk of contaminating the granules, it does have chemical affinity for granules made up of silica soot particles, and under the temperature conditions of the oven, that leads to the granules adhering to the walls of the crucible. In practical terms, adhesion makes it difficult to recover the granules in simple manner, and any mechanical action runs the risk of adding contamination.